Copy Control Information is added to digital data to protect the copyright of the data. In DTCP (Digital Transmission Content Protection, refer to non-patent document 1), for example, the Copy Control Information is recorded in CCI (Copy Control Information) which is transmitted simultaneously with a stream or EMI (Encryption Mode Indicator) that is included in a packet header as two-bit information. The following four categories of copy control are assigned by the two-bit information: Copy-free (permitted to copy without restriction), No-more-copies (permitted to copy once), Copy-one-generation (prohibited from dubbing), Copy-never (prohibited from copying).
A stream assigned to Copy-free is permitted of both storage and recording. Here, the “storage” means a recording process by which the stream can be played back only by the apparatus recording the stream, while “recording” means a recording process by which the stream can be played back by other apparatuses. A stream assigned to Copy-one-generation can also be permitted of both storage and recording by reassigning the CCI of the stream as No-more-copies. A stream assigned to No-more-copies is neither permitted of storage nor recording. On the other hand, although a stream assigned to Copy-never is not permitted of recording, storage is permitted for the time designated as Retention Time in Retention State Field transmitted simultaneously with a stream only in the case to display the stream with a certain delay. The Retention State consists of three-bit information, and the relationship of the three-bit information and Retention Time is as shown in FIG. 9.
FIG. 9 shows the relationship between Retention State (three-bit information) and Retention Time. As is shown, a stream can be stored for the durations assigned as Retention Time corresponding to each status of Retention State.
Similar operations are being carried in BS/CS digital broadcastings (for example, refer to non-patent document 2, non-patent document 3). In these cases, information corresponding with CCI or EMI is digital recording control data contained in digital copy control description elements which are transmitted simultaneously with a stream. The digital recording control data are two-bit data, and the states of “copy permitted without restriction”, “copy permitted only one generation”, “copy prohibited” are assigned to the value of the data. A category equivalent to “copy permitted only once” is not assigned in broadcasting field but is interpreted as “copy prohibited”.
Durations of temporary storage are designated by retention mode (one-bit data) and retention state (three-bit data). This means that even a stream assigned to “copy prohibited” is permitted to be stored temporarily for the duration designated by retention state in the case retention mode is assigned as zero. The function “displaying a stream with a certain delay” described above is typically effectuated as a time shift function of a recorder equipped with an internal hard disk drive(s).
Conventionally, a stream has been assigned to one of three states when it is stored. The first state is “copy permitted without restriction” for storing a Copy Free stream. The second state is “copy permitted only once” for storing a Copy Once stream. The third state is “temporary storing” for storing a Copy Never stream. A stream assigned to “temporary storing” is required to disable its content to be played back after a designated duration has lapsed.
Concerning the disabling of a content to be played back, in patent document 1, for example, a method is disclosed to disable a content to be played back by generating an encryption key based on random numbers that change with time and by canceling the retention of random numbers after a given duration.
In patent document 2, a recordable play-back equipment with a storage (AV-HDD) equipped with an internal timer is disclosed, which has a function to delete copy-prohibited data when a designated time has lapsed.
Patent Document 1: Japanese Laid-Open Patent Publication No. 2003-333030
Patent Document 2: Japanese Laid-Open Patent Publication No. 2003-140976
Non-patent document 1: Digital Transmission Content Protection Specification Volume 1 (Informational Version)
Non-patent document 2: Association of Radio Industries and Businesses (ARIB), “Operational Guidelines for BS/Broadband CS Digital Broadcasting”, ARIB TR-B15, Ver. 2. 9, Vol. 8, February 2004
Non-patent document 3: Association of Radio Industries and Businesses (ARIB), “Service Information for Digital Broadcasting System” ARIB STD-B10, Ver. 3. 8, Feb. 2004